Process for increasing the electri



Patented Aug. 27, 1935 PROCESS. FOR INCREASING THE ELECTRI- CAL RESISTANCE OF FUSED MAGNESIUM OXIDE Robert J. Sutton, Wooster, Ohio, and Jonathan R. Fritze, Western Springs, Ill., assignors to Edison General Electric Appliance Company,

Inc.,

Chicago, Ill., a corporation 01 New York No Drawing. Application October 11, 1932, Serial No. 637,364

6 Claims.

Our invention relates to electrical insulation and has for its object the provision of improved electrical insulation and process of making it. More specifically our invention relates to improvements in fused magnesium oxide.

One common use of this material is to electrically insulate the resistance conductors and terminals of sheathed electric heaters of the type described and claimed in United States Patcnt No. 1,367 ,341 to C. C. Abbott, dated February 1, 1921. As there described, heaters of this type comprise a resistance conductor inclosed by a metallic sheath and insulated from it by a densely compacted layer of magnesium oxide, usually in powdered form.

It is desirable that the insulating layer between the conductor and the sheath be comparatively thin so that the conductor is in intimate thermal relation with its inclosing sheath. However, it is important that the insulating layer possess suificient'electrical resistivity to thoroughly insulate the heater, and so, the thickness of the layer must not be reduced to such an extent that the insulation of the heater would be dangerously weakened.

In one of its aspects our invention contemplates the provision of an improved process for treating fused magnesium oxide whereby its electrical resistivity is materially increased.

One of the beneficial results of our process is that for a given insulation requirement the thickness of the insulating layer is materially reduced, or in other words, for a given thickness of insulating layer, the electrical resistivity is considerably increased.

In another of its aspects our invention contemplates the provision of an improved process for treating fused magnesium oxide whereby its stability in the presence of moisture is increased.

In accordance with our invention the fused magnesium oxide is heated or fired for a certain period of time, whereby certain chemical action or physical alteration takes place which increases the electrical resistivity of the oxide and its stability in the presence of moisture.

Fused magnesium oxide used in electric heaters usually is made from magnesium carbonate, which when suitably treated by heat is reduced to magnesium oxide, and carbon dioxide which passes off as a gas. The magnesium oxide thus formed is fused in a suitable furnace or crucible at some suitable comparatively high temperature such as 5000 F. The fused magnesium oxide' which is made in this manner usually is in lump form, and to render it available for use, it is subjected to a crushing or grinding or like mechanical operation to reduce it to a comparatively fine powder. It is this powder which heretofore has generally been used to insulate electric heaters of the sheathed type. in order to increase the electrical resistivity of the fused magnesium oxide and moreover to increase its stability in the presence of moisture we heat or fire the magnesium oxide at a suitable temperature and for a certain period of time. While both the temperature at whichthe oxide is fired and the firing period may be varied, we have found that quite satisfactory results are obtained when the magnesium oxide is heated to a temperature of approximately 2100 F. and held at this temperature for approximately six hours. Preferably the oxide should be heated to this temperature at a comparatively slow rate, and likewise after this heating period the oxide should be allowed to cool at a comparatively slow rate. Satisfactory results will be obtained if the temperature of the oxide is gradually increased from room temperature to the heating temperature of 2100 F. through a heating period of thirty or forty hours or more and after heating, the oxide is allowed to cool gradually for thirty or forty hours or more until it attains room temperature. These values may vary but for the most satisfactory results it is important that the magnesium oxide be brought up to its heating temperature at a comparatively slow rate, and likewise, that it be allowed to cool comparatively slowly after being heated.

The heating temperature and the period of heating may be varied somewhat from the values mentioned above, but it is believed that the most satisfactory results will be obtained if the oxide is heated at an elevated temperature materially in excess of 1000 R, such as 2000-2300 F. and that it be heated for approximately six hours at this temperature. If a temperature lower than this range of 2000-2300 F. be used the oxide should 'be heated fora longer period of time.

On the other hand, if a higher temperature be used the heating period should be shortened accordingly.

Preferably the fused magnesium oxide will be heated in a laboratory or commercial kiln used to fire porcelain or terra cotta or the like. Suitable containers, such as saggers, may be used to hold the magnesium oxide while it is being fired.

The heating operation may be carried out either in an oxidizing or in a reducing atmosphere. When such impurities as iron and carbon are present, it is preferable to heat in an oxidizing atmosphere whereby these impurities are removed by As previously pointed out, 5

' oxidation. In such cases we have found that very satisfactory results are obtained by heating in air.

It is not understood definitely what happens to the fused magnesium oxide by the above treatment. tion, chemical alteration, or an 'intemal rearrangement of molecules is not known. It is known, however, that the electrical resistivity of the oxide is materially increased by our process. In certain instances, the electrical reslstivity'of the oxide has been increased more than 300%, even as high as 1000%. Moreover, it is known that the stability of the magnesium oxide in the presence of moisture is definitely increased.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. A process for increasing the electrical resistivity of fused magnesium oxide which comprises heating said magnesium oxide in an oxidizing atmosphere for approximately six hours at a temperature of approximately 2000 F.-2300 F.

2. A process for increasing the electrical resistivity of fused magnesium oxide which comprises heating said magnesium oxide from thirty to forty hours so that its temperature is gradually increased from room temperature to approximately 2000 F.-2300 F., maintaining the temperature of said oxide at approximately 2000 F.- 2300" F. for approximately six hours and then gradually cooling said magnesium oxide to room temperature through a period of approximately thirty to forty hours.

3. A process for increasing the electrical resistivity of fused magnesium oxide and its sta- Whether the change is a surface condi-' bility in the presence ofmoisture whichcomprises heating said fused-magnesium oxide at a temperature of approximately 2000 F.-2300 F.

' for approximately six hours.

4. A process for increasing the electrical resistivity of fused magnesium oxide and its stability in the presence of moisture which comprises heating said fused magnesium oxide in an oxidizing atmosphere at a temperature of approximately 2100 F. for approximately six hours.

5. A process for increasing the electricalresistivity of fused magnesium oxide and its stability in the presence of moisture which comprises heating. said magnesium'oxide for thirty to forty hours so that its temperature is gradually increased from room temperature to approximately 2100 F., heating said oxide approximately at 2100 F. for approximately six hours and then allowing said magnesium oxide to gradually cool to room temperature through a period of thirty 

